The present invention relates to a condensation heat exchanger associated—directly or indirectly—with a burner, in particular a gas or fuel-oil burner, and equipped with a gas/air heat recuperator.
An exchanger of this type is designed particularly to be fitted to a domestic gas boiler for supplying a central-heating system and/or for providing water for household use.
Advantageously, but not necessarily, the heat exchanger that is the subject of the invention is of the type that comprises a jacket delimiting an enclosure accommodating at least one bundle made up of a tube or tubes of flattened cross section, of the type described in document EP-B-0 678 186, to which reference may be made if required.
Said document describes a heat exchanger element that consists of a tube made from a material that is a good conductor of heat and in which a heat-transfer fluid, for example water to be heated, is intended to circulate.
The tube is wound as a helix and has an oval, flattened cross section, the major axis of which is substantially perpendicular to the axis of the helix, and each turn of the tube has planar faces that are separated from the faces of the adjacent turn by a gap of constant width, this width being substantially smaller than the thickness of said cross section, the space between two adjacent turns also being fixed by means of spacers consisting, for example, of bosses formed in the tube wall.
Said document also describes heat exchangers that include several elements such as those described above, arranged in different ways in the various embodiments disclosed.
An exchanger element designed in this way is capable of ensuring a very efficient heat exchange between very hot gases on the one hand, which may be generated directly by a burner mounted in the enclosure or originate from an external source, which sweep over the tubular element, and, on the other hand, the fluid, such as water, to be heated, which circulates inside the tubular element.
In fact, as it passes across the gap between the turns, in an approximately radial direction, the flow of hot gases comes into contact with a relatively large surface area of the wall of the exchanger element.
The jacket forming the condensation units of the type described above, just like the tube or tubes, may be made from metal, particularly stainless steel.
It is, however, advantageously produced from plastic, as mentioned in French patent applications No 02/12848 (Oct. 16, 2002) and No 03/00775 (Jan. 24, 2003).
In this case, the exchanger includes means for mechanical containment of the bundle, in the axial direction of the latter, capable of absorbing the thrust forces that result from the internal pressure of the fluid circulating therein and which tends to deform the walls thereof, preventing these forces being transferred to the jacket.
The two roles hitherto fulfilled by the jacket, namely serving as the enclosure for circulation and evacuation of the hot gases and for the collection and evacuation of the condensates and, in addition, guaranteeing the mechanical robustness of the tube bundle, are thus separated.
The condensation heat exchanger according to the invention is associated with a gas or fuel-oil burner, and comprises at least one tube bundle through which a fluid to be heated, in particular cold water, circulates and which is mounted inside a gas-impermeable jacket for example made from plastic, said tube bundle being exposed to hot gases generated by the burner, while the jacket has a flue-gas evacuation sleeve.
According to a preferred embodiment, the condensation heat exchanger according to the invention comprises two coaxial tube bundles placed end-to-end, one of which acts as primary exchanger and the other of which acts as secondary exchanger, each of these bundles consisting of a tube or of a group of tubes arranged end-to-end, forming a helical coil, in which the wall of the tube(s) is produced from a material that is a good conductor of heat and has a flattened, oval cross section, the major axis of which is perpendicular or approximately perpendicular to the axis of the helix, while the width of the gap separating two adjacent turns is constant and, particularly, smaller than the thickness of said cross section, said bundles being mounted inside a gas-impermeable jacket.
The burner is arranged inside the primary exchanger.
Means are provided in order to circulate at least one fluid to be heated, in particular cold water, inside the tube(s) forming said bundles, the above mentioned jacket said jacket having a burnt-gas evacuation sleeve, the exchanger being arranged such that the hot gases supplying the exchanger generated by the burner flow radially, or approximately radially, through said bundles, passing through the gaps separating its turns, a deflection plate also being interposed between these two bundles and arranged in such a manner that said hot gases first flow through the primary exchanger, flowing through the gaps separating its turns from the inside to the outside, then the secondary exchanger, flowing through the gaps separating its turns, this time from the outside to the inside, after which they are evacuated to the outside via the above-mentioned sleeve.
As in the device shown in FIG. 18 of document EP-B-0 678 186 cited above, the deflection plate advantageously consists of a disk made from refractory, thermally insulating material based, for example, on ceramics, mounted at the free end of the burner. This disk is provided at its periphery with a thermally insulating seal that is applied against the inside of the bundle.
Changes in regulations relating to the emission of polluting gases emanating from domestic heating units increasingly encourage boiler manufacturers to design units that minimize emissions that are likely to pollute the atmosphere.
Condensing boilers functioning with a blown-gas burner ensure good combustion and make it possible to recover the latent heat contained in the flue gases by virtue of the condensation phenomenon, which reduces the quantity of pollutant emissions as compared to conventional boilers.
Nevertheless, the exit temperature of the flue gases is limited by the temperature of the fluid, particularly the water, to be heated. Thus, if this water enters the exchanger at a temperature of 50° C. and exits therefrom at a temperature of 70° C. it is not possible to envisage lowering the temperature of the flue gases to a value below 50° C. In practice, this value will ultimately be close to 70° C.
It is indeed the temperature of the water entering the unit that limits the temperature of the flue gases escaping therefrom with their polluting particles. The condensation limit also depends on the temperature of the incoming water to the extent that if this temperature is equal to or higher than the dew point of the flue gases there is no condensation.
In point of fact, in the case of flue gases resulting from combustion of natural gas, this dew point is of the order of 55° C.
The flue gases generated by the exchanger are thus diffused into the atmosphere at a relatively high temperature, producing an unattractive white plume (on account of the water vapor conveyed by it), with polluting particles and with a heat loss that detracts from the unit's overall performance.
In order to palliate these drawbacks, it is known to add an additional gas/air exchanger onto the exchanger, the function of which is to preheat the (fresh) combustion air captured outside before conveying it to the burner, this being with the aid of the still-hot flue gases that are exiting the exchanger, i.e. the burnt gases.
A system of this type is described, for example, in U.S. Pat. No. 4,640,232.
By virtue of this arrangement, it is the temperature of the outside air and no longer the temperature of the water that determines whether the water vapor present in the flue gases will condense. As this temperature is below the dew point, almost total condensation is achieved such that the expelled flue gases are cold and practically free from polluting particles, the latter being present in the flue gas condensate.
Furthermore, as the air reaching the burner is warm, combustion is improved and the performance is better.
The air captured outside is generally moist and polluted, particularly in an urban atmosphere. As it passes into the additional gas/air exchanger, condensation of the water vapor it is conveying also occurs when it encounters the warm walls of the exchanger. The water droplets that form trap the polluting particles, which are therefore contained within the incoming-air condensate. The additional gas/air exchanger thus also fulfills the function of scrubbing the combustion air.
In this way, clean air that has been rid of its impurities is used as combustion air, which also improves the quality of combustion and significantly reduces fouling of the principal exchanger.